Thanks for your thoughts, until now I never really considered
exporting storage from the SAN to my domU’s. I can definitely see the advantage
here, using the SAN snapshot utilities, it most cases it can be automated. I am
interested in how you would accomplish similar functionality to the SAN
snapshot, using LVM snapshots (let’s say lvm snapshot support worked well).

Okay. For CLVM it probably makes the most sense to run
one big volume group across your cluster, but there's also the option of
running a non-clustered LVM on each Dom0 host. The latter would only
work for you however if you don't require Xen migration.

I see 3 options for central storage in a Xen cluster, each with
their own drawbacks:

1) Run a single clustered volume group across all hosts,
containing one or more PV's from your shared storage.

2) Run a non-clustered volume group on each host, each with a
distinct PV carved out of your shared storage.

3) Export storage for each host individually from your SAN, i.e.
rely completely on your SAN for volume management. With this you don't
need LVM at all.

Both 1) and 3) allow you to use Xen migration. 2) is feasible
if you don't need to migrate guests online.

Our problem with 1) is snapshot support, and that we could not get
pvmove to work acceptably well. (We had to make the entire volume group
inactive before pvmove would even run--I'm not sure if it is expected, or
what we did wrong.)

We've tried and failed at 1), and will now be attempting 3).
This gives us a lot of flexibility on a storage appliance that supports
snapshots. I'd still like to have pvmove work so we could migrate online
from one SAN to another, if needed, but I haven't been able to get it to
work acceptably well.

Also I thought I had read that snapshots are not supported by a
clustered LVM? That would be difficult for us too, as we are
relying on snapshots for a backup mechanism.

Yes to both. Right now the cluster is running GFS and I
can migrate VM’s between the nodes.

This question is coming up because I have been trying to do a
snap shot and I realized the snapshot is stored on the Volume Group that the LV
is located on. I did not realize this and I cannot do a snapshot
because I did not leave enough space in each of the Volume Groups for each of
the VM’s.

If I am installing multiple Xen VM’s in a cluster with shared
iSCSI space with Logical Volumes for each virtual machine should I put each LV
in its own logical volume group or should I use one logical volume group for
all of the LV’s?